The invention relates to a motor vehicle with all wheels driven via differential gearing or the like, wherein a control device samples the rotational speed of the wheels via sensors and which--in order to achieve synchronism between driving wheels, actuates locks associated with the differential gearing for predeterminable holding times when rotational-speed differences of the wheels exceed threshold values which are permanently predetermined or predetermined in dependence on the driving condition. The control can actuate the locks of the differential gearing, or differential gears, or individual brakes associated with the driving wheels, and/or a clutch arrangement connecting the connectable all-wheel drive to the drive shafting of the vehicle to control the speed of the driven wheels.
In addition to vehicles with permanent all-wheel drive, there are vehicles in which the all-wheel drive is manually or automatically connected.
For automatic connection, viscous clutches are often used in which the torque transmitted depends on the differences in rotational speed between the clutch input and output. When the rotational-speed difference is exceeded, the amount of transferable torque rises steeply. This makes it possible for the vehicle to behave like a rear wheel-driven (or also front-wheel-driven) vehicle when driving on a non-slip base. The all-wheel drive only becomes effective under difficult driving conditions.
However, it is disadvantageous if the all-wheel drive becomes effective with a permanently predetermined difference in rotational speed between front and rear wheels and accordingly it is impossible to take into consideration special driving conditions.
For this reason, there are automatically connectable all-wheel drives which are operated by means of a control device which is responsive to rotational wheel speeds sensors and to other parameters such as, for example, steering angles or the like. This basically provides an opportunity for controlling the all-wheel drive in an extremely flexible manner. For example, the all-wheel drive can only be connected within a limited speed range and when the vehicle is allowed to drive exclusively in a predetermined driving mode (e.g. with rear-wheel drive or with front-wheel drive at relatively high speeds). It is also possible to connect the all-wheel drive at different driving speeds only after different threshold values for the rotational-speed differences between front and/or rear wheels are exceeded. As a result, the manageability of the vehicle can be considerably improved in boundary situations.
In principle, the same applies to differential locks, for example those which lock a central differential between front axle and rear axle in an all-wheel drive, or the differential of a driving axle in vehicles having only one driven axle or all wheel drive. Although locks can be basically constructed as viscous clutches, arrangements in which the locks or their locking degree are controlled by means of a control device in dependence on the rotational wheel speeds (or the differences of the rotational wheel speeds) are superior with respect to the possible variability of the control system.
In the case of vehicles having only one driven axle, instead of providing a differential lock, one can separately control wheel brakes in dependence on the rotational wheel speeds (or on differences between the rotational wheel speeds) by means of the control device in order to brake a driving wheel which is spinning and thus rotating very quickly compared with the remaining vehicle wheels. As a result, an increased torque is automatically transmitted via the differential to the other driving wheel so that a desired synchronism of the driving wheels can be produced.
In all systems in which the wheel brakes, and/or the differential locks, and/or the clutch arrangement controlling the connectable all-wheel drive, are actuated by means of a control device, it is advantageous if the sensors, necessary for controlling the rotational wheel speeds, already exist. Because vehicles equipped with such elaborate driving systems usually also have a service brake with an anti-blocking system, the necessary sensors are available without adding them. For others, control devices with computer support can also be used for other tasks when there is adequate computer capacity in such vehicles.
It is then an object of the invention to further develop a motor vehicle of the above general type in such a manner that a particularly good adaptation to respective driving conditions is rendered possible.
According to the invention, this object is achieved by having the control device actuate the lock or locks, the brake or brakes, and/or the clutch device after partial or complete disengagement of the latter (test disconnection) in dependence on a variable time interval. The time interval elapses after the partial or complete disengagement to a new occurrence of the threshold value for progressively increased holding times, and/or during a predeterminable time interval, even when a reduced threshold value or reduced threshold values occur.
The invention is based on the general finding that especially difficult roadway conditions (slippery) must be present if a new intervention becomes necessary, immediately after an intervention of the control device in the distribution of the drive power to the permanent, or connectable drive wheels. In order to be able to perform a sensitive control, the respective intervention can initially occur during a very brief holding time which is then followed by a test disconnection. That is to say, the previously actuated lock, brake and/or clutch arrangement are disengaged and subsequently actuated for an increased holding time if the rotational wheel speeds (or differences of these), which occur during the test disconnection, exceed predeterminable threshold values. Due to the fact that the holding time is increasingly extended up to a maximum value, one avoids possible excitation of oscillations which are unwanted in the drive shafting.
Additionally or alternatively, the response sensitivity of the control device is increased after an intervention of the latter so that, under certain circumstances, there is no complete test disconnection of the brake, lock and/or clutch arrangement which was in each case previously actuated. Instead the latter are already actuated when rotational speed differences between the wheels of the vehicle are beginning or are still relatively slight.
In the case of an automatically connected all-wheel drive, the drive wheels are thus not even completely disconnected from the drive shafting during a test disconnection. Instead, under certain circumstances, only a reduced torque is transmitted to the connected driving wheels during the test disconnection. This makes it possible to prevent the continuously driven driving wheel(s) from slipping excessively during the test disconnection and the traction power of the vehicle dropping to a comparative extent.
A differential lock operating with force locking can be controlled in the same manner. If rotational-speed differences, (which exceed a previously reduced threshold value) occur at the driving wheels associated with the respective differential, even with partial detachment of the differential lock (i.e. with reduced force locking) the differential lock is again controlled to become fully effective. In this case there is no complete test disconnection.
Even if the synchronism of driving wheels is achieved by separate actuation of the wheel brakes, it is possible to reactuate an actuated wheel brake more strongly after only partial reduction of the braking force and when a previously reduced threshold value of a rotational-speed difference of the driving wheels is already reached or exceeded during the partial detachment of the said wheel brake. This ensures that the driving wheel which is rotating more slowly in each case is continuously supplied with a certain driving torque.
In a particularly preferred embodiment of the invention, the length of the time interval (in which the control device responds to reduced threshold values) depends on the frequency or the succession in time of the threshold-value transgressions. As a result, the brakes or locks or clutch devices respond in a particularly sensitive manner without significant delay in slippery driving conditions.
It is also possible that the control device continuously checks which rotational accelerations or decelerations of the vehicle wheels occur and continuously updates corresponding measurement values and stores these for a predetermined period of time. As long as no relatively large rotational-speed differences between the vehicle wheels have occurred in the rotational accelerations or decelerations registered, the registered rotational acceleration or deceleration represents a measure of the minimum coefficient of friction existing on the respective roadway. If a relatively large rotational-speed difference between the different wheels has occurred on acceleration of the vehicle, or if the anti-blocking system has responded during braking of the vehicle, the rotational accelerations or decelerations of the wheels, represent a measure of the top limit of the coefficient of friction of the respective roadway. If then, the control device changes the threshold values of the rotational-speed differences (occurring between the vehicle wheels at which the control device intervenes in the distribution of the drive power to the permanent or connectable drive wheels) analogously to the magnitude of the rotational accelerations or decelerations of the wheels occurring, the drive system can be matched to the coefficients of friction of the respective roadway. This is true since the control system already intervenes in the distribution of the drive power to the permanent or connectable driving wheels with relatively low rotational-speed differences between the vehicle wheels on a slippery base.
In a further advantageous development of the invention, it is preferable to change the threshold values in dependence on speed and in dependence on the steering angle, the threshold values being reduced with increasing value of the product of speed and steering angle. This results in the control device intervening early in the distribution of the drive power to the permanently driven or connectable driving wheels in tight curves, for example when driving through mountain passes. This prevents sudden or abrupt increased driving torques becoming effective at the driving wheels which rotate more slowly during the intervention of the control device in the distribution of the driving power.
This has the following significance: when driving in curves, a driving wheel can spin particularly during acceleration. This is equivalent to the spinning driving wheel losing its ground adherence and thus also its cornering force while the slowly rotating driving wheel still works with ground adherence and correspondingly existing cornering force. Preventing a driving torque from being suddenly (abruptly) applied to the slowly rotating driving wheel, effectively prevents the slowly rotating driving wheel from also losing its ground adherence or cornering force due to the intervention of the control device in the distribution of the driving power.
In a particularly preferred embodiment of the invention, rotational-speed differences between front and rear wheels as well as rotational-speed differences between the wheels of opposite vehicle sides are registered. It is thus possible, when driving quickly in curves, to control the distribution of the drive power to the permanently driven or connectable driving wheels primarily in dependence on rotational-speed differences between the wheels of opposite vehicle sides. This is done by increasing threshold values for the permissible rotational-speed differences with increasing driving speed and increasing steering angles in order to take into account different track radii and rotating speeds of the wheels when driving in curves.
To avoid a change in the driving characteristics which may surprise the driver in certain circumstances, for example the transition from understeering to oversteering behavior, it may be suitable if the control device opens any rear-axle differential lock which may have been actuated (and/or the lock of the central differential between front and rear axle) only when the steering has reached its straight-ahead position or an adjoining position.
To facilitate the starting on a slippery base, the control device can leave the beginning position of the locks, brakes and/or clutch arrangements, which have been switched to be effective during a starting attempt, within a predeterminable time interval during the previous starting attempt. This provides that during the subsequent starting attempt, the system does not wait until rotational-speed differences occur between the wheels of the vehicle and, instead, a synchronism of the driving wheels is immediately obtained so that starting can occur with the least-possible slippage.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.